Heating coils employed in electrical furnaces operative at exceedingly high temperatures are typically supported by ceramic cores such as grooved plates or cylinders wherin the heater is supported and often confined throughout its entire length by the ceramic structure. The weight of the ceramic support structure constitutes a major percentage of the overall heater assembly mass by reason of the amount of ceramic necessary for support of the heating element and the inherent density of the ceramic material. As a result of the relatively massive amount of ceramic material present in a heater assembly of conventional construction, the heater exhibits a high thermal inertia which limits the rapidity with which a change of temperature can be accomplished. The response of such conventional heaters to temperature control is thereby limited by the relatively slow thermal response of the heater structure.
The function of the ceramic core in each of these prior heaters is to support and contain the electrical heating element. The core may be composed of a cylindrical rod or a circular or rectangular plate having a plurality of longitudinal re-entrant slots or grooves formed in the peripheral surface thereof and running the length of said surface. These grooves, due to the limitations imposed by the ceramic material, are necessarily of small diameter and will expose at the maximum one fifth the surface area of the electrical heating element itself. The ceramic core therefore effectively shades at least 80% of the direct radiation emitted by the coil of the product, thus providing a low standard of emissivity. This low emissivity in turn promotes a substantial differential in temperature between the product and the heating element, causing inefficiency and shorter heating life.
There are many other problems with heater coils set in grooves. When using a flat ceramic plate with a plurality of longitudinal parallel grooves formed in the plane of one surface, and placed above and below the materials and product being heated, the lower flat heater collects particles which must be cleaned or the heater will short circuit. The use of large amounts of ceramic material, small restraining grooves and relatively thin heating wire combine to yield poor tensile strength, an inefficient level of thermal inertia and a large temperature differential between the heater and the product.
Other heater configurations employ a solid heater rod which in one well-known configuration is wound in a circular helical configuration with ceramic spacers interposed between helix turns to maintain spacing. This type of heater depends on its radial arch for support, and the heater must support not only the weight of the rod, but also that of the ceramic spacers. At high temperatures, such rod heaters tend to sag, and in addition, the heating surface thereof is shaded by the presence of the ceramic spacers.
Examples of prior devices are shown in U.S. Pat. Nos. 2,870,308; 3,651 304; 3,673,387; 3,783,238 and 3,798,417. A high temperature heater which overcomes the deficiencies of the prior art is the subject of copending application Ser. No. 622235, filed of even date herewith, entitled HIGH TEMPERATURE FURNACE HEATER and assigned to the same assignee of the present invention.